The invention concerns a multifunctional catalyst, its method of preparation and its use in treating exhaust fumes from internal combustion engines, more specifically for converting the carbon monoxide, hydrocarbons and nitrogen oxides contained in these fumes.
Exhaust fumes from internal combustion engines essentially contain nitrogen, water, carbon dioxide, oxygen and hydrogen, which are considered as relatively harmless, and carbon monoxide, hydrocarbons and nitrogen oxides, which are known to pollute the environment. The preferred method of eliminating these pollutants is catalytic promotion of the oxidation-reduction reactions between them or with relatively harmless compounds, essentially to form nitrogen, water and carbon dioxide, all of which are relatively harmless to the environment.
A catalyst is described as multifunctional, when it is capable of simultaneously promoting the conversion of a plurality of pollutants to relatively harmless compounds.
Catalysts for treating exhaust fumes may be in the form of spheres or monolithic blocks. In this case the catalyst comprises an inert metal or ceramic substrate, which is coated with a layer generally of alumina, cerium oxide and precious metal oxides.
The richness of an air-fuel mix supplying an engine is defined as the ratio between the stoichiometric air/fuel ratio and the actual air/fuel ratio. A richness level of 1 signifies that the air-fuel mix is strictly stoichiometric. A richness of over 1 signifies that the mix contains an excess of fuel relative to the quantity of air present.
The state of the art is illustrated by U.S. Pat. No. 4,526,886, British Patent 1,262,076, U.S. Pat. No. 4,162,262 and French Patent 2,165,056.
More specifically, cerium oxide is known to stabilise alumina and to have an oxygen storing capacity (J. S. SHYU, W. H. WEBER and H. S. GANDHI, J. Phys. Chem. 1988, 92, 4964). It increases the activity of catalysts for a reaction wherein a gas is converted to water.
The oxygen storage capacity is the ability--which cerium and some other materials have--to be oxidised in an oxidising exhaust gas and, in being reduced, to oxidise reducing compounds even when the exhaust gas becomes a reducing agent.
Catalysts not containing any cerium but containing uranium have already been described for treating exhaust fumes from internal combustion engines. Thus British patent GB 1 262 076 describes a catalyst containing iron, nickel and uranium deposited on alumina. But this catalyst does not contain any precious metals, a fact which greatly limits its effectiveness in converting the pollutants.
U.S. Pat. No. 4,526,886 describes a catalyst containing uranium, platinum and rhodium dispersed on an inorganic oxide carrier with a large specific surface area. The catalyst is more particularly effective when it operates in an exhaust gas with a mean richness of over 1, or with a mean richness of 1 but with strong oscillations around that value. Furthermore it remains very active even in the presence of oxide of sulphur.
However, known catalysts containing uranium have the disadvantage that their activity is much degraded when they are subjected to prolonged high temperature treatment.